What preservative is added to samples to adjust the pH to less than 2?

• A. Hydrochloric acid
• B. Acetic acid
• C. Sulfuric acid
• D. Nitric acid

Answer: (C)

Lowering the sample pH to below 2 is a preservation step to stop biological activity and keep analytes stable during storage and transport. A strong mineral acid is needed to achieve a rapid, durable acidification without adding volatile components. Sulfuric acid is commonly used because it is a strong, non-volatile acid that stays in solution, reliably brings pH to well under 2, and helps keep metals in solution by preventing hydroxide precipitation. Among the acids listed, sulfuric acid best fits this preservation role for wastewater samples in many testing protocols. Hydrochloric acid could lower pH but may introduce chloride interference and be more corrosive; acetic acid is weak and won’t consistently reach pH below 2; nitric acid is strong and also used in some methods, but sulfuric acid is the standard choice in this context.